This invention relates to radiation processing for the degradation of materials and specifically to radiation degradation of polytetrafluoroethylene (PTFE).
U.S. Pat. No. 3,766,031 to Dillon discloses a method for radiation processing of polytetrafluoroethylene, wherein the polytetrafluoroethylene is exposed to radiation and thereafter subjected to comminution to reduce the polytetrafluoroethylene to a fine particle powder. This powder is useful as a dry lubricant, for example, in paints and inks. This and other methods for carrying out this technique generally made use of electron beam or cobalt sources for irradiating the polytetrafluoroethylene. The material being irradiated was arranged in trays and exposed to multiple doses of radiation, approximately 2 to 15 MR per pass, so that the temperature of the material does not rise excessively, thereby to avoid discoloration of the material and the possible generation of noxious gases. The total dose is approximately 35 to 150 MR.
The tray irradiation technique has a radiation utilization efficiency of only approximately 35%. Radiation inefficiencies arise from the three factors, which are (1) overscan of trays, (2) gaps between trays and (3) depth-dose characteristics for an electon beam.
The need to assure complete and uniform radiation of a tray requires some overscan by the radiation and an efficiency loss of 5 to 15%. Likewise there is usually some space between trays of material which causes a further 10 to 15% loss of efficiency.
The largest efficiency loss arises out of the fact that the dose received by the material varies with material depth. This variation is illustrated in the graph of FIG. 6. Typically the dose at the surface, designated R.sub.o, is taken as the nominal dose for the material. Beam energy and/or material depth is adjusted so that an equal dose at the opposite surface of the material (Depth D ). Radiation which passes entirely through the product (Area C) is not utilized. Radiation in Area B in excess of the nominal dose R.sub.o is likewise not used. This causes further inefficiency, and in some instances may result in undesired properties of the resultant product. Overall this depth-dose characteristic can cause processing inefficiency of up to 50%.
The prior art technique of slow irradiation of material by subsequent exposures to doses of radiation, occasionally coupled with stirring the material between exposures, tends to cause the production of radiation degraded polytetrafluoroethylene powder to be an expensive, inefficient and time consuming operation.
It is an object of the present invention to provide an apparatus and method for more efficiently producing radiation degraded polytetrafluoroethylene.